Sonic Underground is an animated television series that follows the adventures of Sonic the Hedgehog, and his siblings Sonia the Hedgehog (his sister) and Manic the Hedgehog (his brother). Sonia is a magenta hedgehog, who wears a red and purple dress. Manic is a green hedgehog who is similar to Sonic in appearance beside the fact that he wears a red vest and his first two spines are replaced by spiky hair. It bared little continuity with the often convoluted Sonic universe — including previous animated series, and Sonic comics and games - and, apart from Sonic and Robotnik, few established characters.

The three are the children of Queen Aleena Hedgehog who was the original ruler of Mobius before Dr. Robotnik seized power. As They travel around Mobius they are chased by Dr. Robotnik and his bumbling bounty hunter sidekicks Sleet and Dingo, who pursue the three because of a prophecy made by the Oracle of Delphius (a spoof of the Oracle of Delphi of Ancient Greece) - stating that the Queen's three children will one day find their mother and together end Robotnik's reign, becoming the rulers once more.

The three siblings have magical amulets that not only change into musical instruments but change into weapons when the triplets are in perfect harmony with each other. Sonic's amulet is an electric guitar/gun, Sonia's amulet is a keyboard/gun, and Manic's amulet is a drum kit/earth controller. The three use the amulets not only to fight Robotnik's forces but also as instruments for their underground rock and roll band, "Sonic Underground".